Air Hose Safety
By Ian Speer · Speer Compression Systems (Pty) Ltd, Kalamunda, Australia · www.speer.com.au
Read about some of the safest ways to achieve lowest hose costs while minimising the operating risks to personnel, plant and the environment.
This is the second article of Part 1 of our Technical Series on Compressor and Air Hose Safety. Click here for an outline of the entire Compressor and Air Hose Safety Series.
Although the title of this article refers to air hoses, Ian discusses the risks associated with all flexible hoses on a drill rig; air, hydraulic and fuel hoses.
All drilling rigs use hoses for a variety of reasons ranging from water transfer at low pressure at the low hazard end of the spectrum, through to the high pressure, high temperature hoses filled with compressed air and oil that connect the discharge port of 34 bar oil flooded screw compressors to the separator vessels.
Up until quite recently, hoses have been viewed by many operators as durable items to be replaced only in the event of failure, or when a major rebuild of the plant in question was being undertaken.
The advent of boosters and higher-pressure oil flooded screw compressors has added a range of new hazards with the potential for catastrophic losses in the event of a hose failure.
To date, the industry has embraced a range of restraint measures designed to contain the hose in the event of a failure. (See the next article in this series and future articles in which we discuss hose-sock restraints). In the case of machines operating in hazardous areas such as coal mines and offshore, restraints have been augmented by isolating potential ignition sources such as exhaust manifolds and turbochargers from hoses in such a way, that should a hose carrying flammable material (oil) under pressure fail and the oil be sprayed around the surrounding area it could not impinge on a surface that was hot enough to result in ignition and a subsequent fire.
Such measures may provide comfort to the rig owner but they afford scant protection to personnel working in the vicinity who may well be sprayed with near boiling oil (100-125oC) resulting in serious burns or worse.
Hazards associated with flexible hoses on compressors and drill rigs
The safest way to achieve lowest hose costs while minimising the operating risks to personnel, plant and the environment:
Hazards within the hose
High pressures
The pressure of a liquid within a hose represents a hazard as a result of being physically sprayed or hit forcibly by a stream released through a failed hose. In the case of a gas such as air there is the additional danger of stored energy which is released explosively when a hose fails.
High temperatures
As the working temperature of the fluid or gas being conveyed by the hose increases; the life of the hose is reduced; likewise, the danger of burns to people working near a hose carrying hot materials is higher than if the hose was carrying cold water for example.
Flammable fluids
If the fluid being conveyed under pressure is flammable or supports combustion then the potential for injury increases.
Pulsation within fluid/gas being conveyed
If a hose is subjected to continuous variations in pressure the hose life will be reduced.
Possibly the best example of this is the percussion feed hose on a drifter rig which operates under amazing loads throughout its life.
External hazards around the hose
UV light
Almost every type of material used in the construction of flexible hoses is adversely affected by UV light. The notable exception I can think of is the flexible stainless steel hose used on some boosters and compressor discharge lines as well.
Physical damage
This can occur as a result of handling such as dragging a booster connecting hose across stony ground, people driving over or walking on hoses or more subtly the weight of suspended hoses such as those from a drill head to the mast which must flex almost continuously as well as support their own weight during use.
Ignition sources
Any surface or object capable of igniting a stream of flammable material that escapes as a result of a hose failure.
Ambient temperature
Heat can cause eventual hardening of the elastomers in the walls of hoses. It can also soften the outer layer of a new hose and result in it tearing if it is dragged over rough ground.
Installed position
A hose installed in close proximity to very hot areas, or where it has to support its own weight or where it is bent in a smaller radius than the maker recommends, will not achieve its maximum life.
Vibration and cyclic loading
Hoses installed on equipment which is subject to vibration will chafe and wear through their outer layers leaving the reinforcing braids exposed; which in the case of steel wire reinforcement will start to corrode; leading to premature hose failure.
Possible consequences of a hose failure
Fire
The failure of a hose carrying flammable material in close proximity to an ignition source will likely result in a fire and loss of plant as well as a good chance of injuring crew in the area.
Injury to crew
Aside from burns, the threat of physical injury due to explosive release of energy when a hose connected to a source of high pressure air fails is very high. Much has been done in the area of hose restraint to control this risk.
Falling load or object
In some areas of a rig the working fluid in the hydraulics supports considerable loads. Reputable manufacturers fit hard plumbed anti fall devices in jacks and mast raise cylinders as well as spring applied brakes to winches.
The area of greatest risk is the rotation head which must be free to move rapidly up and down the mast to allow the rig to function correctly. Rapid failure of the hose which holds the head up when the control valves are closed will result in the head and any drilling tools attached thereto free falling down the mast with considerable potential to damage the rig or to injure the crew.
Environmental contamination
Failure of a hose conveying any substance capable of damaging the environment will result in a spill with attendant clean-up costs and potential loss of work.
Hose rating and de-rating
Effect of temperature
All hose makers provide tables setting out the maximum allowable working temperature of each of their types and sizes of hoses. In general, the pressure rating of a hose falls as its operating temperature increases.
Effect of pressure
The life of a hose reduces as the pressure that it is called on to convey increases.
Effect of hose diameter
The larger the diameter of a hose for a given construction, the lower its operating pressure
Effect of hose construction
More layers and different forms of construction within the hose itself measurably alter the working pressure and life expectancy of the hose.
In addition, the actual type of elastomers used in the construction of the hose has a significant effect on the durability of the hose.
Risk management matrix
Other ideas for consideration
In addition to the risk management strategies there are a couple of proven methods of improving safety around hoses;
Rotate hoses in booster duty
In the case of hoses connecting a compressor to a booster and the booster to the drill rig, one can use the simple idea of always using the new hose on the high-pressure booster discharge and then after its allocated life, given it has not been damaged, this hose is then used on the lower pressure booster feed application, after which it is discarded PRIOR to failure.
Perforation of high pressure air hoses
The leading manufacturers of high pressure air hoses perforate the outer elastomeric cover of the hose to allow any air and accompanying moisture that finds its way through the lining then into the metal braids to be released to the atmosphere rather than to form bubbles which result in corrosion of the reinforcing wire and eventual failure of the hose.
Conclusion
Many of the preceding ideas are very simple and can be adopted quickly at modest cost. By focusing on the higher risk areas plant safety can be measurably improved.
Other areas will involve real expenditure and the payback will be slower but no less real coming in the form of lower accident claims thus resulting in lower insurance premiums.
Acknowledgements
The majority of the material has been derived from observations of the best practices of many companies over some years. This article will have been successful if a single injury or serious loss is avoided as a result of adoption of one or more of the ideas expounded herein.